This invention relates to detectors. More particularly, but not exclusively, this invention relates to direct conversion Quadrature Detectors for use in demodulating FSK modulated radio signals, and to radio receivers incorporating such detectors.
In a conventional zero IF FSK Receiver, shown in FIG. 1, an incoming radio signal on Node 1 is applied to two channels, an in-phase channel I, and a Quadrature-phase channel Q. Both channels are substantially identical and comprise a respective mixer 2, 3, channel filter 4, 5, and a limiting amplifiers 6, 7. A local oscillator 8 is applied directly to the in-phase mixer 2 and via 90.degree. phase shift circuit 9 to the Quadrature-Phase mixer 3. The limiting amplifiers 6, 7 remove the amplitude information from the signals and produce digital output signals containing only phase information. These signals are applied to a detector 10. The output of detector 10 is passed through limiting amplifier 11 to produce a detected base-band signal at its output 12. If necessary, amplifier 11 may incorporate a post-detection filter, not shown. Channel filters 4 and 5 serve to reject unwanted spurious signals from the output 5 of the mixers, and both I and Q channels are ideally identical so as to avoid any additional phase shift. The two signals applied to the inputs of detector 10 therefore always lead or lag each other by 90.degree..
Such prior art circuits have proved satisfactory, but require the use of a highly stable local oscillator LO locked to the carrier frequency of the signal to be received. In practice this means the use of a high quality and relatively expensive crystal oscillator. Such a crystal oscillator can represent a significant part of the expense of a receiver especially in applications such as paging systems.
One way of avoiding the requirement to use a highly stable crystal is to use an AFC (Automatic Frequency Control) loop which locks itself to the carrier of the received signal. The absence of an intermediate frequency signal in direct conversion receivers poses difficulties in the use of an AFC loop for frequency control. The present invention arose from an attempt to provide an improved receiver.